This invention relates to container covers and more particularly comprises a disposable plastic lid having new and improved stacking features that enable a number of identical lids to be arranged in a stack without compacting or jamming.
There are a very wide variety of disposable plastic lids used in the food packaging industry, and these lids are generally provided with some form of stacking facility which enables the lids to be stacked with other identical or substantially identical lids without compacting or jamming even when axially directed loads are imposed on them. Very frequently the stacking facilities take the form of undercuts to provide a positive interference between the engaging portions of adjacent lids. Undercuts have an obvious disadvantage, namely, they make it difficult to strip the lids from the molds after they are thermoformed. The more pronounced the undercut (which provides greater stacking interference), the more difficult are the forming and stripping operations. In accordance with this invention, undercuts in the principal stacking facility are eliminated.
The lid of the present invention is provided with a circular closure wall and a depending skirt. A rim cavity is formed at the junction of the circular wall and skirt, which receives the container rim to form a seal when the lid is placed on the container. A pair of spaced annular ribs are formed in the closure wall adjacent its periphery, and the space between the ribs defines a raised rib or inverted trough. Extending upwardly from the trough are symetrically spaced bosses which are wider than the trough so as to extend into the parallel ribs. When like ribs are stacked one upon the other in the upright position, the upwardly extending bosses in the lower lid engage the bottoms of the parallel ribs of the next upper lid in the stack to prevent the lids from compacting or jamming together. Consequently they are easily separated for use.
This particular configuration has several advantages. First, because the stacking facility defined by the ribs, trough and bosses is free of undercuts, the mold is not difficult to fabricate, and the lids are easy to strip from the mold during manufacture. Second, the stacking facility is free of any substantial downwardly extending projections that would extend into the contents of the container to which the lid is applied. Third, because the bosses are effectively shortened by the intersection by the trough, there is less thinning of the material during the thermal forming operation. That is, the mouth to depth ratio of the draw in the mold during forming is reduced so that the bosses are not weakened or thinned excessively. Fourth, the upwardly extending bosses are not sealed at the bottom by the ribs in the next lower lid in a stack. Consequently, the bosses are vented and no vacuum is formed which would resist lid separation. Fifth, the interruption of the bosses by the trough assists in the evacuation of air from the mold during forming so as to eliminate air distortion.